自然降雨条件下干化土壤水分恢复试验

土壤干燥化是制约黄土丘陵半干旱地区生态环境可持续发展的瓶颈问题,自然降雨是当地土壤水分补充的唯一来源。为探究自然降雨条件下干化土壤的水分恢复状况,在陕北米脂试验站构建野外10 m深大型地下土柱,2014—2019年间利用CS650-CR1000型土壤水分自动监测系统和BLJW-4小型气象观测站对土壤水分、气象状况进行连续定位观测,分别对次降雨、月降雨、年降雨条件下的干化土壤入渗恢复深度进行研究。结果表明:黄土区降雨分布的阶段性和年际不均衡性是直接影响深层干化土壤水分入渗的主导因素,年内降雨可以划分为3个阶段:降雨匮乏阶段(上年11—次年3月)、降雨过渡增加阶段(4—6月)、降雨丰沛阶段(7—10月)。试验期间,能够促进深层干化土壤水分恢复的有效降雨(入渗深度大于50 cm)发生次数为56次,有效降雨量1 455.20 mm,分别为6a总降雨次数和总降雨量的16.23%、64.68%。月尺度、年尺度条件下,逐月、逐年降雨入渗深度均随降雨量增加呈二次函数增大变化,累积降雨入渗深度则随时间延续持续增大,对深层干化土壤水分恢复起决定作用。至2018年12月,累积降雨入渗深度达到1 000cm。以农地土壤水分为标准,2014—2019年自然降雨条件下干化土壤完全恢复深度分别为140、180、300、600、700、700cm,完全恢复程度依次可达14%、18%、30%、60%、70%、70%。研究结果对于探讨半干旱黄土区在自然降雨条件下林地深层干化土壤水分恢复规律,制定合理措施促进干化土壤水分恢复,以及深入干化土壤水文循环机理研究具有重要意义。

Experimental study on moisture recovery of dried soil under natural precipitation condition

Abstract: Drought and water shortage have posed a great threat to the ecological and agricultural development in the semi-arid loess hilly region. Local vegetation normally determines the dynamic balance between natural precipitation and crop water demand. Since a long-term negative cycle of soil moisture can lead to serious soil desiccation, water supplies have a great impact on the ecological environment and sustainable agriculture. This study aims to explore the moisture recovery of dried soil under natural rainfall conditions. An underground soil column was built up to 10 m in the Mizhi test station located in the north of Shaanxi Province, China. The soil moisture and the meteorological parameters had been observed by the CS650-R1000 soil water automatic monitoring system and BLJW-4 meteorological observatory from 2014 to 2019. The results show that the distribution of natural rainfall in the loess areas was obviously uneven at various stages in different years. The rainfall was divided into three stages in one year: the deficit rainfall (from last November to March this year), the increasing rainfall (from April to June), and the abundant rainfall (from July to October). On a multi-year scale, high-, flat- and dry-water year alternately occurred. The distribution of rainfall directly dominated the infiltration in the deeply dried soil. During the 6-year observation period, there were 56 times effective rainfall events with a precipitation of 1455.20 mm, which could infiltrate to 50cm or below. Specifically, the effective rates were 16.23% and 64.68%, respectively. On the month and year scale, the rainfall had an independent influence on the infiltration in one month or year. The infiltration depths fluctuated with the precipitation in terms of the quadratic function. The maximum monthly precipitation (209.60 mm) occurred in July 2016, with the maximum depth of monthly infiltration (400 cm), and the maximum yearly rainfall (590.80 mm) occurred in 2016, with the maximum infiltration depth of 400cm. Nevertheless, the cumulative infiltration depths continued to increase with time, due possibly to the interaction of each other under multi-month and multi-year rainfall conditions. Particularly, the cumulative depths of rainfall infiltration reached 1000 cm by December of 2018. The entire profile of 1000cm dried soil was obtained at the different levels of water recovery. The complete recovery depths of dried soil under the natural rainfall conditions from 2014 to 2019 were 140, 180, 300, 600, 700, and 700 cm, respectively. Compared with the soil moisture of farmland, the complete recovery increased 14%, 18%, 30%, 60%, 70%, and 70%, respectively. The findings are greatly significant to explore the water recovery of deeply dried soil under natural rainfall conditions, thereby formulating some reasonable measures for the hydrological cycle in semi-arid loess areas.